Metal alkyl borphydride polymerisation initiators, polymerisable compositions, and uses thereof

ABSTRACT

Metal alkyl borohydrides are used as initiators of polymerisation, particularly in adhesive compositions for bonding a wide range of substrates including low surface energy substrates such as polyolefins. As described, the metal alkyl borohydrides are of the formula I or II.  
                 
 
     wherein R 1  is C 1 -C 10  alkyl,  
     R 2  and R 3 , which may be the same or different, are H, D, C 1 -C 10  alkyl or C 3 -C 10  cycloalkyl, phenyl, or phenyl-substituted C 1 -C 10  alkyl or C 3 -C 10  cycloalkyl, provided that any two of R 1 -R 3  may optionally be part of a carbocyclic ring, and  
     M +  is a metal ion.  
     In particular, alkali metal trialkyl borohydrides are used, the alkali metal salt being selected from: Lithium triethylborohydride, Sodium triethylborohydride, Potassium triethylborohydride, Lithium tri-sec-butylborohydride, Sodium tri-sec-butylborohydride, Potassium tri-sec-butylborohydride, and Lithium triethylborodeuteride. Other exemplified compounds which are less effective on low surface energy substrates include Lithium 9-borabicyclo [3.3.1]-nonane (9BBN) hydride, Lithium thexylborohydride, Lithium trisiamylborohydride and Potassium trisiamylborohydride.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is related to the application entitled“Polymerisation Initiators, Polymerisable Compositions, and Uses Thereoffor Bonding Low Surface Energy Substrates” filed on even date herewithas International application No. (Agent's file PM978PCT) claimingpriority from European Patent Application No. 00650166.2 filed on 23Oct. 2000; the contents of the said International application areincorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates to novel polymerisation initiators,polymerisable compositions and uses thereof, particularly uses asadhesives, sealants, surface coatings, moulding resins, and compositematrices. In one aspect, this invention is concerned with acrylicadhesive compositions, particularly compositions for bonding a widerange of substrates including low surface energy substrates such aspolyolefins to each other or to other substrates such as metals.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

[0003] The chemistry of organic boron compounds has been studied indetail (see “Comprehensive Organic Chemistry” by Barton D. and Ollis W.D., Volume 3, Edited by Jones D. N., Pergamon Press, 1979,Part 14). Theuse of organoboranes such as the trialkylboranes includingtriethylborane and tributylborane for initiating and catalyzing thepolymerisation of vinyl monomers is well known. However suchorganoborane compounds are known to be flammable in air so that thecompounds and compositions containing them require special handling andthe compositions have poor shelf stability (see for U.S. Pat. No.3,236,823 Jennes et al., and the Background section of U.S. Pat. No.5,935,711 Pocius et al., at column 2).

[0004] Certain boron alkyl compounds and their use as initiators ofpolymerisation are described in a series of patents of Wolfgang Ritterassigned to Henkel KgaA, including U.S. Pat. Nos. 4,515,724, 4,638,092,4,638,498, 4,676,858 and 4,921,921 (hereinafter referred to as “theRitter patents”). However the adhesive systems developed from thesepatents require the manufacture of trialkyl boranes from long chainfatty acids.

[0005] A series of patents of Skoultchi or Skoultchi et al. disclose atwo-part initiator system for acrylic adhesive compositions in which thefirst part includes a stable organoborane amine complex and the secondpart includes a destabilizer or activator such as an organic acid or analdehyde (U.S. Pat. Nos. 5,106,928; 5,143,884; 5,286,821; 5,310,835 and5,376,746).

[0006] Japanese patent publication No. S48-18928 describes a method foradhering polyolefin or vinyl polymers using an adhesive obtained byadding trialkylboron to a vinyl monomer or vinyl monomer and vinylpolymer. Examples of trialkylboron include triisopropylboron,tri-n-butylboron, tripropylboron and tri-tert.-butylboron.

[0007] U.S. Pat. No., 3,275,611, Mottus et al. describes a process forpolymerising unsaturated monomers with a catalyst comprising anorganoboron compound, a peroxygen compound and an amine complexing agentfor the boron compound. Use of the polymerisation products as adhesivesis not discussed.

[0008] It is well known that the bonding of polyolefin substrates andother low surface energy substrates causes particular difficulties.Attempts have been made to overcome this problem by extensive andexpensive substrate surface preparation, for example by oxidation,plasma treatment, corona treatment or flame treatment, or by priming thesurface with a high surface energy primer. However it is desired todevelop adhesive compositions which will bond low surface energysubstrates without such surface preparation.

[0009] U.S. Pat. No. 5,539,070 of Zharov et al., assigned to MinnesotaMining and Manufacturing Company and a series of patents of Alphonsus V.Pocius or Pocius et al. also assigned to Minnesota Mining andManufacturing Company, including in particular U.S. Pat. Nos. 5,616,796,5,621,143, 5,681,910, 5,684,102, 5,686,544, 5,718,977, 5,795,657 and5,935,711 describe organoborane amine complexes which can be used insystems that initiate the polymerisation of acrylic monomers incompositions useful for bonding low surface energy plastics substratessuch as polyethylene, polypropylene and polytetrafluoroethylene. PCTPublication No., WO 99/64528 also of Minnesota Mining and ManufacturingCompany describes low odour polymerisable compositions comprisingmonomer blends and organoborane amine complex initiators. However thesesystems require the preparation of trialkyl borane amine complexes toachieve the desired performance and shelf stability. The manufacture ofsuch complexes is an undesirably complicated process. Furthermore thepresence of the amines results in cured adhesives that have a tendencyto become yellow in colour on ageing.

[0010] PCT Publication No. WO 01/44311 of the Dow Chemical Company,published after the priority date of this application, also describesamine organoborane complex polymerisation initiators in bondingcompositions for low surface energy substrates.

[0011] PCT Publication No. WO 01/32716 of 3M Innovative PropertiesCompany, published after the priority date of this application,acknowledges that while complexes of an organoborane and an amine may beuseful in many applications, certain problems may arise due to the useof amine complexing agents in such conventional complexes: for example,when the complexes contain a primary amine, adhesives prepared therefrommay be prone to discolouration, such as yellowing; furthermore, whenincluding reactive diluents, such as aziridine—functional materialsdescribed in PCT Publication No. WO 98/17,694, for example, incompositions containing the complexes, the diluents may prematurelyreact with protic amines (i.e. those amines in which a nitrogen atom isbonded to at least one hydrogen atom) in such complexes, prematurelydecomplexing the organoborane initiator. WO 01/32716 therefore proposesa complex of an organoborane and a complexing agent comprising at leastone hydroxide or alkoxide, particularly a complex represented by theformula.

[0012] wherein R¹ is an alkyl group having 1 to about 10 carbon atoms;R² and R³ may be the same or different and are selected from (i.e., theyare independently selected from) alkyl groups having 1 to about 10carbon atoms and phenyl-containing groups; the value of “v” is selectedso as to provide an effective ratio of oxygen atoms of the alkoxidesand/or hydroxides to boron atoms in the complex; each R⁴ isindependently selected from hydrogen or an organic group (e.g., an alkylor alkylene group); M^((m+)) represents a countercation (comprising amonovalent cation, such as a Group IA metal (e.g., lithium, sodium andpotassium) cation or onium, or a multivalent cation, such as a Group IIAmetal (e.g., calcium and magnesium)cation); n is an integer greater thanzero; and m is an integer greater than zero. Particular complexingagents comprise those having a countercation comprising a cationselected from sodium, potassium and tetraalkylammoniums.

[0013] When any R⁴ is hydrogen, the complexing agent is said to compriseat least one hydroxide. When any R⁴ is an organic group, the complexingagent is said to comprise at least one alkoxide.

[0014] According to WO 01/32716, the complexing agent (i.e., thehydroxide or alkoxide) is used in the form of a salt. That is, thecomplexing agent is stabilized by a suitable countercation such that thecomplexing agent is capable of complexing the initiator. Thus, in theFormula, M^((m+)) represents a countercation that stabilizes thecomplexing agent, not a cation that forms an ionic compound with theorganoborane initiator.

[0015] It is stated in WO 01/32716 that hydroxides and alkoxides providestrong coupling to organometallic initiators, such as organoboranes,with the resulting complexes having excellent oxidative stability. Thus,the use of complexing agents comprising at least one hydroxide,alkoxide, or mixtures thereof is said to be particularly beneficial.

[0016] It is also stated in WO 01/32716 that such a complex is a tightlyco-ordinated salt formed by association of a Lewis acid (the initiator)and a Lewis base (the hydroxide or alkoxide). This indicates that theoxygen atom of the alkoxide or hydroxide is bonded or coordinated to theboron atom of the initiator.

[0017] It is known to use tetraorganylborate salts oftetraalkylammonium, sodium or lithium as photopolymerisation initiatorsin photocurable compositions for imaging materials (see, for exampleU.S. Pat. No. 4,950,581 Koike et al, assigned to Fuji Photo Film Co.Ltd.; U.S. Pat. No. 6,110,987 Kamata et al. assigned to Showa DenkoK.K.; and U.S. Pat. No. 6,171,700 Sugita et al. assigned to Showa DenkoK.K. and Showa Highpolymer Ltd.). Such tetraorganylborate salts do nothave a boron-hydrogen bond. The distinction between boron-hydrogencompounds, triorganylboranes and organoborate salts is well illustratedin “Comprehensive Organic Chemistry” by Barton & Ollis (Ed. D NevilleJones) cited above, Volume 3, in which Chapters 14.2, 14.3 and 14.4 aredevoted to them. The photopolymerisable compositions of theabove-mentioned patents are not intended for use as adhesives, sealantsand the like. Photopolymerisation systems operate by absorption of lightenergy, and the characteristic feature of such systems is that they havelight-absorbing ability. It is not predictable whetherphotopolymerisation initiators will be suitable for other cure systemsor for bonding substrates having particular surface energy conditions.In particular, the above-mentioned patents concerningphotopolymerisation initiators do not provide any disclosure or teachingrelating to the bonding of low surface energy substrates.

[0018] Despite the work of many researchers in this field, there is aneed for polymerisation initiators which reduce the problems describedabove and which provide alternatives to the systems available hitherto.It is desirable to provide initiators which are commercially availablecompounds, thereby removing complicated manufacturing processes for suchcomplexes from formulators' concerns, and/or to provide initiators whichare relatively easy to handle compared to compounds of the prior artThere is a need also for commercially acceptable compositions forbonding low surface energy substrates such as polyolefins, and for endusers to have a variety of such compositions which achieve that resultthrough different technical strategies.

SUMMARY OF THE INVENTION

[0019] This invention relates to the use of metal alkyl borohydrides asinitiators of polymerisation. The applicants are not aware of anydisclosure or teaching of the use of ionic compounds of this kind aspolymerisation initiators. The term “hydride” as used herein includesall of the isotopes thereof including deuterides. A metalalkylborohydride has at least one alkyl group attached to the boronatom. In “Comprehensive Organic Chemistry” by Barton & Ollis, citedabove, alkyl borohydrides are treated under the heading of“Organylhydroborates” (see para. 14.2.6.4).

[0020] In accordance with one aspect of the invention, the metal alkylborohydrides used as initiators of polymerisation are selected from thegroup consisting of compounds of formula I or II:

[0021] wherein R¹ is C₁-C₁₀ alkyl,

[0022] R² and R³, which may be the same or different, are H, D, C₁-C₁₀alkyl or C₃-C₁₀ cycloalkyl, phenyl, or phenyl-substituted C₁-C₁₀ alkylor C₃-C₁₀ cycloalkyl, provided that any two of R¹-R³ may optionally bepart of a carbocyclic ring, and

[0023] M⁺ is a metal ion.

[0024] An alkyl group may suitably have 1-6 carbon atoms, for example2-4 carbon atoms. An alkyl group may be straight-chain or branched,although straight chain is prefeered for bonding low surface energypolymers. A carbocyclic ring may be bridged by the boron atom.

[0025] Suitably at least two, and desirably three, of R¹-R³ are C¹-C₁₀alkyl.

[0026] Preferably the metal alkyl borohydride is of the formula I.

[0027] Due to the presence of the boron-hydrogen bond, the metal alkylborohydrides are reducing agents, which provides a significant potentialadvantage over known organborane initiators. If and when alkylboranesare freed from the borohydride, they would be prone to oxidation andcould produce peroxides. In that case, the presence of reducing agents(unreacted borohydride) would be beneficial in facilitatingdecomposition of these peroxides to produce radicals for polymerisation.

[0028] The presence of air or oxygen is desirable at the time ofinitiation of polymerisation e.g. when the two parts of a two-partcomposition are mixed.

[0029] The metal alkyl borohydrides used in the present invention arecommercially available or are readily prepared by available methods.They are available or can be prepared in solutions which are easy tohandle as compared to known trialkyl boranes, i.e. the solutions are notpyrophoric and only require the exclusion of moisture. The metal alkylborohydride initiators do not use amines which have been used in thepast to form covalent complexes with organoboranes. Once theorganoborane of such complexes has reacted in the polymerisablecomposition, the amine can cause yellowing of a cured composition afterageing. A reduction or elimination of yellowing is a particularadvantage for adhesives which are to be used in locations whereappearance is important, e.g. in automobile manufacture. Amines may alsogive rise to odour problems. Compositions containing the metal alkylborohydrides have good shelf stability.

[0030] If lithium hydride is reacted with trimethylborane in solventssuch as tetrahydrofuran, monoglyme, diglyme, etc., either lithiumtrimethylborohydride (1:1 adduct, LiMe₃BH) or lithiumhexamethyldiborohydride (1:2 adduct, LiMe₃BHBMe₃) may be formed (seeHerbert C Brown et al. in “Addition Compounds of Alkali Metal Hydrides.14. The Reaction of Trialkylboranes with Lithium Trialkylborohydrides”,J.A.C.S/99:19/Sep. 14, 1977). The formula I or II as used hereinencompasses 1:2 adducts of this kind, which may be represented by theformula Ia or IIa:

[0031] wherein R¹, R², R³, and M⁺ are as defined above. Part or all ofthe metal alkyl borohydride used in the present invention may be a 1:2adduct of this kind.

[0032] The invention particularly relates to the use of metal alkylborohydrides as polymerisation initiators in adhesive compositions orcoating compositions.

[0033] The invention provides a polymerisable composition, whichincludes:

[0034] a) at least one free-radically polymerisable monomer component,and

[0035] b) an effective amount of an initiator system for initiatingpolymerisation of the free-radically polymerisable monomer, saidinitiator system comprising a metal alkyl borohydride, particularly acompound of the formula I or II as defined above.

[0036] The polymerisable composition may suitably be a two-partcomposition in which the at least one free-radically polymerisablemonomer component is provided in one part and the metal alkylborohydride is provided in the other part. Alternatively the compositioncould be a one-part composition with suitable stabilisation andactivation systems such as a moisture-latent acid or oxygen-latent acid.In a further alternative, the metal alkyl borohydride could be providedas a primer, in which the metal alkyl borohydride is applied to asubstrate separately from the adhesive composition.

[0037] The polymerisable monomer or monomers may suitably be one or more(meth)acrylic monomers.

[0038] In one aspect the invention provides a two-part polymerisableadhesive composition, which includes:

[0039] part A) an effective amount of a polymerisation initiatorcomprising a metal alkyl borohydride, and a carrier which is inert tothe metal alkyl borohydride;

[0040] part B) at least one (meth)acrylate monomer, optionally with atoughener, acidic monomer, filler and/or thickener.

[0041] Part B may suitably be a (meth)acrylic component. The terms(meth)acrylic and (meth)acrylate are used synonymously herein withregard to the monomer and monomer-containing component. The terms(meth)acrylic and (meth)acrylate include acrylic, methacrylic, acrylateand methacrylate.

[0042] Although the invention is not limited by any theory, it isbelieved that on mixing the initiator component with a polymerisablemonomer component containing acid (or on release of acid in a systemusing a latent acid), the metal alkyl borohydride reacts with the acidto release triorganylboranes and metal salts of the acid, and possiblyhydrogen.

[0043] Therefore in one aspect the invention provides a polymerisablecomposition, which includes

[0044] a) at least one free-radically polymerisable monomer component,

[0045] b) an effective amount of an initiator system for initiatingpolymerisation of the free-radically polymerisable monomer, saidinitiator system, comprising a metal alkyl borohydride, and

[0046] c) an effective amount of a compound that is reactive with themetal alkyl borohydride for liberating the organoborane to initiatepolymerisation of the at least one-free radically polymerisable monomer.

[0047] Suitably the compound that is reactive with the metal alkylborohydride is present in part B of a two-part polymerisable acrylicadhesive composition as described above.

[0048] Moisture is excluded from contact with the metal alkylborohydride until polymerisation has been initiated. Suitably part A ofthe two-part adhesive composition is packaged in a moisture-free andmoisture-impermeable applicator or other container.

[0049] The adhesive compositions disclosed herein are useful for bondinga large range of substrates including metals, plastics and glass tosimilar or different substrates and indeed they give better performancethan the previously discussed adhesives on many of these substrates,especially glass. The compositions of the invention are particularlyuseful for bonding low surface energy substrates e.g. those having asurface energy of less than 45 mJ/m², more particularly polyolefinsincluding polyethylene and polypropylene,acrylonitrile-butadiene-styrene and polytetrafluroethylene, orrelatively low surface energy substrates such as polycarbonate, tosimilar substrates, to each other, or to different substrates includingmetals, other plastics and glass.

[0050] In a particular aspect the invention relates to an adhesivecomposition as defined above for bonding a low surface energy substrateto a similar or different substrate. In such composition it is preferredthat at least two of R₁ and R₂ are straight chain C₁-C₄ alkyl.

[0051] Furthermore the invention provides a method for bonding asubstrate to a similar or different substrate, wherein the methodcomprises applying an adhesive composition as defined above to at leastone of the substrates, bringing the substrates together and allowing thecomposition to cure. In one aspect, the invention provides a method asdefined above for bonding a low surface energy substrate such as apolyolefin substrate to a similar or different substrate.

[0052] In particular the invention provides a method for bonding asubstrate, particularly a low surface energy substrate, to a similar ordifferent substrate, wherein the method comprises mixing parts A and Bof a two part adhesive composition immediately prior to use in order toinitiate polymerisation, applying the mixed adhesive composition to atleast one of the substrates, bringing the substrates together andallowing the composition to cure by completion of the polymerisationinitiated on mixing of the two parts A and B.

DETAILED DESCRIPTION OF THE INVENTION

[0053] According to one aspect of the invention M is an alkali metal ionsuch as lithium, sodium, potassium, or caesium, particularly lithium,sodium or potassium, although metallic elements in the second row of thePeriodic Table such as barium, magnesium or calcium may be useable, asmay transition metals such as copper, iron or cobalt.

[0054] R¹-R³ may suitably be the same or different alkyl group and eachmay suitably be a C₁-C₆ alkyl group, particularly a C₂-C₄ alkyl group.Desirably the three alkyl groups R¹-R³ are the same group. Large orbulky alkyl groups are considered to be less likely to provide desirablyactive initiators, as will be understood by those skilled infree-radical chemistry. A cycloalkyl group may suitably be C₅-C₆ group.

[0055] Examples of suitable alkali metal salts in which R¹—R³ are thesame alkyl group include the following: Lithium triethylborohydride,Sodium triethylborohydride, Potassium triethylborohydride, Lithiumtri-sec-butylborohydride, Sodium tri-sec-butylborohydride, Potassiumtri-sec-butylborohydride, Lithium trisiamylborohydride, Potassiumtrisiamylborohydride and Lithium triethylborodeuteride. A particularlysuitable example is Lithium triethylborohydride.

[0056] An example of a compound in which R² is an alkyl group and R¹ andR³ are H is Lithium thexylborohydride. An example of a compound in whichR¹ and R³ form part of a cyclic ring is lithium9-borabicyclo[3.3.1]-nonane (9BBN) hydride.

[0057] Desirably at least one of R² and R³ is a C¹-C₁₀ alkyl group.Suitably one or both of R² and R³ may be phenyl. Desirably not more thanone of R² and R³ contains a phenyl group. It will be understood by thoseskilled in the art that a phenyl group may be substituted in the ring byone or more substituents which do not affect the activity of thecompound of formula I or II as a polymerisation initiator. Suchring-substituents include C¹-C₁₀ alkyl, for example C₁-C₆ alkyl,particularly methyl.

[0058] The above-identified compounds are commercially available fromvarious suppliers such as Sigma-Aldrich Ireland Limited of Dublin 24,Ireland e.g under trade names such as Super-hydride, Selectride, andSuper-deuteride, or Callery Chemicals of Evans City Pa., USA under tradenames such as CalSelect. Other suitable compounds include the metalborohydrides analogous to the boron compounds as described in the Ritterpatents such as, for example diisopinocamphenylborane,dicyclohexylborane, and diisoamylborane.

[0059] One group of exemplary compounds includes the following: Lithiumtriethylborohydride, Sodium triethylborohydride, Potassiumtriethylborohydride, Lithium tri-sec-butylborohydride, and Lithium9-borabicyclo [3.3.1]-nonane (9-BBN) hydride.

[0060] The metal alkyl borohydride may suitably be used in a solventsuch as tetrahydrofuran, diglyme, dibutyl ether, toluene or ahydrocarbon solvent, for example at 1 molar concentration, preferablynot more than about 1.5 molar, although the person skilled in the artwill select a suitable concentration depending upon the solvent used.

[0061] The quantity of metal alkyl borohydride may suitably be such asto provide 0.01% to 5% by weight, particularly 0.01% to 2% by weight,such as 0.01 to 0.6% by weight, of boron in the total composition. Forease in handling, the metal alkyl borohydride is suitably used in asolution in a concentration up to about 1.5M, such as about 1M.

[0062] The free-radically polymerisable monomer may be selected fromolefinically unsaturated systems such as acrylates, methacrylates,styrene, maleate esters, fumarate esters, unsaturated polyester resins,alkyd resins, thiol-ene compositions, and acrylate, methacrylate, orvinyl terminated resins including silicones and urethanes. Amongsuitable acrylates and methacrylates are those used in polymerisablesystems such as disclosed in U.S. Pat. No. 4,295,909, U.S. Pat. No.4,018,851, U.S. Pat. No. 4,963,220 to Baccei et. al., and U.S. Pat. No.4,215,209 to Ray-Chaudhuri et. al. or polyfunctional methacrylates,silicone diacrylates and polyfunctional acrylated urethanes of the typeknown to be useful in formulating adhesives e.g. as disclosed in U.S.Pat. No. 4,092,376 to Douek et. al.) or a thiol-ene (e.g. as disclosedin U.S. Pat. Nos. 3,661,744, 3,898,349, 4,008,341 or 4,808,638). Thecontents of the above-mentioned patents are incorporated herein byreference.

[0063] Suitable monomers include monofunctional acrylate andmethacrylate esters and substituted derivatives thereof such as hydroxy,amide, cyano, chloro, and silane derivatives. Such monomers includetetrahydrofurfuryl (meth)acrylate, methyl (meth)acrylate, ethyl(meth)acrylate, isobornyl methacrylate, hydroxyethyl methacrylate,hydroxypropyl (meth)acrylate, butyl acrylate, n-octyl acrylate,2-ethylhexyl (meth)acrylate, decylmethacrylate, dodecyl methacrylate,cyclohexyl methacrylate, tert.-butyl methacrylate, acrylamide,gamma-methacryloxypropyl trimethoxysilane, 2-cyanoethyl acrylate,3-cyanopropyl acrylate, tetrahydrofurfuryl chloroacrylate, and glycidyl(meth)acrylate.

[0064] Blends of two or more monomers, particularly two or more(meth)acrylate monomers, may desirably be used, the choice of monomersin the blend being determined by the end use application of thecompositions, as known to those skilled in the art.

[0065] It has been found that the metal alkyl borohydrides are effectiveinitiators with or without an acid added to the composition. However theinclusion of an acid (which term includes a latent acid) in the monomercomponent (Part B) is desirable. For certain substrates and/or certainpolymerisation speeds, the presence of an acid may be required. In othercases, acidic residues, for example in the monomers, may be sufficient.Alternatively, or in addition, the metal alkyl borohydride, being areducing agent, may react with another compound in the composition(particularly in Part B of a two-part composition), which compound isopen to reduction, for example an ester such as a methacrylate ester, ora metal in a higher valency state. Reaction of the metal alkylborohydride with such a compound liberates the alkylborane to initiatepolymerisation of the polymerisable composition. In the case of a 2-partcomposition, the reaction takes place after mixing of the two parts, onecontaining the metal alkyl borohydride and the other containing the acidor other compound reactive therewith.

[0066] The acid when added may suitably be a weak acid. Lewis acids maybe used (see U.S. Pat. No. 5,539,070 to Zharov et al., the contents ofwhich are incorporated herein by reference). The pKa of the weak acidnormally is no lower than about 0.5 with the desirable limit being about0.9. The upper limit normally is about 13, or less, such as 11.5.However, carboxylic acids which have a pKa of up to about 8, such as 6or 7, are particularly suitable.

[0067] The carboxylic acids may contain one or more carboxyl groups,suitably 1 to 4, and more preferably 1 to 2, carboxyl groups. Suitablealiphatic carboxylic acids ordinarily include C₁₋₁₈ chains, such asC₁₋₁₀ monocarboxylic acids.

[0068] Suitable acids may be monobasic or polybasic. Typical but notlimiting examples of suitable acids are formic acid, acetic acid,propionic acid, maleic acid, malic acid, fumaric acid, acrylic acid andcopolymers thereof, methacrylic acid and copolymers thereof, pyruvicacid, itaconic acid, nadic acid, benzoic acid, phthalic acids, cinnamicacid, trichloroacetic acid and saccharin. It is particularly suitable touse an acidic monomer which can itself be polymerised, so that it isbound into the cured polymer composition, for example a part-ester of apolyfunctional acid wherein the ester group contains a free-radicallypolymerisable component, particularly a (meth)acrylic halfester of adifunctional acid such as maleic, fumaric or succinnic acid e.g.2-(meth)acryloyloxyethyl maleate, 2-(meth)acryloxyethyl fumarate or2-(meth) acryloxyethyl succinate. The effective amount of the acid orthe acid residue of an acidic monomer or residue resulting frompreparation of a monomer (if present) is suitably within the range fromabout 0.1 to about 20%, particularly from about 0.1 to about 10%, suchas from about 0.5 to about 5% based on the weight of the polymerisablecomposition. The amount of an acidic monomer (if present) is suitablywithin the range from about 0.1% to about 25%, such as about 0.5% toabout 15%, based on the weight of the polymerisable composition.

[0069] The acid may be present as a latent acid, particularly a maskedcarboxylic acid compound hydrolyzable on contact with moisture, such asan acid anhydride, as described in EP-A-0 356 875 and U.S. Pat. No.5,268, 436, the contents of which are incorporated herein by reference.A latent acid may be used in Part B of a two-part composition, in whichthe acid is released on mixing the two parts, or in a one-partcomposition.

[0070] The metal alkyl borohydride is desirably used with a carrierwhich is non-reactive with the metal alkyl borohydride. Most suitably,the carrier should be liquid, should be capable of supporting andcarrying the borohydride, should be moisture-free, non-reactive with abase and not susceptible to free-radical polymerisation although it mayreact with polymeric acids of the adhesive component to contribute tothe polymer network. The carrier may suitably be a solvent for the metalalkyl borohydride.

[0071] It is important that the metal alkyl borohydride and carriershould not react together or start to cross-link, causing a change inviscosity, before polymerisation of the total polymerisable compositionis initiated. Suitable carriers include aziridine-functional materialswhich are described as diluents in PCT Publication No. WO 99/64528,which in turn refers to PCT Publication No., WO 98/17694, thecounterpart of U.S. Pat. No. 5,935,711, the contents of all of which areincorporated herein by reference. When used, the metal alkyl borohydrideis carried by (e.g., dissolved in or diluted by) an aziridine-functionalmaterial or a blend of two or more different aziridine-functionalmaterials in the initiator component. As indicated above, theaziridine-functional material should not be reactive toward the metalalkyl borohydride. Also advantageously, the aziridine-functionalmaterial may reduce the flammabilty of the initiator component.

[0072] An “aziridine-functional material” refers to an organic compoundhaving at least one aziridine ring or group,

[0073] the carbon atom(s) of which may optionally be substituted byC₁-C₁₀ alkyl groups, particularly C₁-C₃ alkyl groups.

[0074] Suitable aziridine-functional materials are described in WO98/17694 (U.S. Pat. No. 5,935,711). Polydifunctional aziridines such astrimethylolpropane tris(3-(2-methylaziridino) propionate areparticularly suitable. The aziridine-functional material may also act asa cross-linking agent in the polymerisable composition, e.g. by reactionwith an acid monomer or polymeric acid which may suitably be present inthe composition as described above.

[0075] The aziridine-functional material should be generally soluble inmonomers included in the polymerisable composition, such that the partsof the two-part composition can be readily mixed. By “soluble” is meantthat no evidence of gross phase separation at room temperature (i.e.,about 22° C. to about 25° C.) is visible to the unaided eye. Similarly,the metal alkyl borohydride should also desirably be soluble in theaziridine-functional material, although some heating may be required.Suitably, the aziridine-functional material is a liquid at or near roomtemperature (i.e., within about 10° C. of room temperature) or forms aliquid solution with the metal alkyl borohydride at near roomtemperature.

[0076] Other suitable carriers include liquid polyethers, liquidpolyethers capped with non-reactive groups being groups which are notsusceptible to free-radical polymerisation such as epoxies, liquidpolyesters, polyisoprene or polybutadiene. A thickened solvent couldalso be used as a carrier. Polytetrahydrofuran could be used as bothsolvent and carrier. The quantity of carrier may suitably be in therange from about 5% to about 50% by weight, for example 5% to 25% byweight, particularly 5% to 10% by weight, of the total composition. Itis an advantage of the present invention that a wider range of carrierscan be used than with an amine-containing initiator system, e.g. anepoxy which could react with the amine.

[0077] The metal alkyl borohydride and carrier (optionally withthickener) are usually contained in Part A—the initiator component—of atwo-part composition. If desired, Part A may also contain a complexingagent or sequestering agent, e.g. a calixarene or a polyether orpolythioether, e.g. a crown ether or thiocrown ether respectively, withaffinity for the metal in the alkali metal borohydride.

[0078] The monomer containing component—Part B of a two-partcomposition—may suitably include a toughener to improve the impactresistance and peel resistance of the bond while maintaining adhesivestrength. Suitable tougheners include elastomeric materials such aspolybutadiene rubbers, polyisoprene (.e.g. available under the tradename Kratan), acrylonitrile-butadiene-styrene (e.g. available under thetrade name Hycaror or as core-shell polymers under the trade nameBlendex), or polystyrenes. For bonding polyolefins it is desirable touse a core-shell polymer. The use of core-shell polymers in(meth)acrylate-based compositions is described, for example, in U.S.Pat. Nos. 4,536,546 and 4,942,201 Briggs et al. assigned to IllinoisTool Works. Core shell polymers are suitably graft copolymer resins(e.g. acrylonitrile-butadiene-styrene graft copolymers or othersdescribed in the above-mentioned patents of Briggs et al.) in the formof particles that comprise rubber or rubber—like cores or networks thatare surrounded by relatively hard shells. In addition to improving theimpact resistance of the bond, core-shell polymers can also impartenhanced spreading and flow properties to the composition (see WO99/64528). These enhanced properties include a reduced tendency for thecomposition to leave an undesirable “string” upon dispensing from asyringe-type applicator, or sap or slump after having been applied to avertical surface. The quantity of toughener may suitably be in the rangefrom about 1% to about 40% by weight, particularly about 5% to about 25%by weight, of the total composition.

[0079] The composition may optionally further comprise oxidising agents,reducing agents, thickeners, fillers, non-reactive colourants andpigments, metal salts (particularly transition metal salts) and freeradical polymerisation stabilisers. The optional additives are used inan amount that does not significantly adversely affect thepolymerisation process or the desired properties of polymerisationproducts made thereby. Ingredients for a photopolymerisation system arenot required and may be excluded.

[0080] Suitable reducing agents may be, but are not limited to,acetylphenylhydrazine, tetramethylthiourea or thiocaprolactam. Suitableoxidising agents may be, but are not limited to, peroxides andhydroperoxides.

[0081] Polymeric thickeners may be present in the compositions in aminor amount, up to about 50%, and may be thickeners such as a polymeror prepolymer of low or high molecular weight. Suitable polymericthickeners are a commercially available methacrylate polymer sold byE.I. du Pont de Nemours and Company, under the trademark Elvacite, aswell as styrene-methyl methacrylate co-polymers and polybisphenol Amaleate or propoxylated bisphenol-A-fumarate polyester (sold under thetrademark Atlac). It is also possible to add inert filling materialssuch as finely divided silica, fumed silica (treated or untreated),montmorillonite, clay, bentonite and the like. The use of micronizedsilica would result in a paste-like thixotropic composition. Polymericthickeners or other thickeners such as silicas may suitably be presentas a thickener for the carrier in Part A of a two-part composition.

[0082] It is an advantage of the present invention that a broader rangeof thickeners can be used than with an initiator system containingamine. Additionally, it is conventional to include in adhesiveformulations certain “inert” fillers such as wood flour, glass fibresand hollow glass spheres, cotton linters, mica, alumina, silica and thelike to modify viscosity, improve impact resistance and for otherpurposes. Such fillers could be incorporated in the formulations of thepresent invention. The quantity of filler is suitably from about 0.5% toabout 20%, for example about 1.0% to about 5.0%, by weight of thecomposition. Small percentages of silane monomers could also be added toincrease moisture resistance, as well as to enhance bonding of anadhesive to glass and similar surfaces. Other substances such as dyes,fire retarders, stabilizers such as quinones and hydroquinones,thixotropes, plasticizers, antioxidants, and the like may also beincluded, although such additives may often be furnished in theprincipal ingredients, making their separate introduction unnecessary.

[0083] It is a particular advantage of the invention that polyolefinmaterials can be used as fillers in the composition. Polyolefin powderssuch as polyethylene powder or polypropylene powder are relativelyinexpensive and in a composition which readily bonds to a polyolefin thefiller becomes securely adhered into the cured composition. Polyethyleneor polypropylene powders can be used with particle sizes in the rangefrom 0.01 mm to 1 mm, particularly 0.02 mm to 0.3 mm, allowing a goodcontrol of the gap between substrates (i.e. the depth of adhesive).Particularly suitable polyethylene powders are commercially availableunder the trade name Microthene. Polyethylene flock and polyolefinchopped fibre can also be used as fillers. The amount of polyolefinfiller may suitably be 0.5% to about 20%, particularly 1.0% to about10%, by weight of the composition.

[0084] Compositions of the present invention may be used as adhesives,sealants, surface coatings, moulding resins and composite matrices, forexample with “fleece” or “padding” materials of glass fibre, carbonfibre, metal fibre, polyethylene or polypropylene fibre or foams, or anycombination of them, particularly in which bonding of a low surfaceenergy polymer is required.

[0085] The compositions may be used in an un-polymerised state, in whichcase polymerisation occurs in situ, or they may be used in apart-polymerised state, in which case polymerisation is completed insitu.

[0086] The components of a two-part composition may suitably be mixedimmediately prior to use in a manner known to those skilled in the art.

[0087] The compositions of the invention are suited for use withconventional, commercially available dispensing equipment for two-partadhesives, for example a dual syringe applicator and a static mixernozzle. In general the compositions are suitable for curing at roomtemperature, i.e. 20-25° without added heat or other energy input,although heat input may be desirable in some instances, for example toaccelerate cure.

[0088] The compositions of the present invention may suitably compriseabout 0.01% to about 5% by weight of the initiator, about 5% to about50% by weight of the carrier (if present), about 0.1% to about 20% byweight of the acid (if present), about 5% to about 85% by weight of thepolymerisable monomer(s) and about 1% to about 40% by weight of thetoughener (if present). The compositions of the invention may suitablyconsist essentially of the ingredients defined in the precedingsentence, together with conventional ingredients such as fillers,thickeners or stabilizers, making up the total composition as 100%.

[0089] In a two-part composition, the parts may suitably be provided ina weight ratio in the range from 1:10 to 1:1 of initiator Component A tomonomer-containing Component B, for example from 1:10 to 1:2, or from1:10 to 1:4.

EXAMPLES

[0090] The following examples will illustrate the invention. In theexamples the lithium triethyl borohydride solution used is commerciallyavailable from Sigma-Aldrich Ireland Limited of Dublin 24, Ireland underthe tradename Super-hydride. The other metal alkyl borohydrides and thetrimethylolpropane tris(3-(2-methylaziridine) propionate used are alsocommercially available from Sigma-Aldrich Ireland Limited. The core-sheutougheners used in the examples are commercially available from GESpeciality Chemicals via Blagden Chemical Specialities Limited, LondonWC1X8NJ, England under the tradenames Blendex 336 and Blendex 360.Additionally the propoxylated Bisphenol-A Fumarate polyester used in theexamples is commercially available from DSM via Philips Duphar (Ireland)Ltd., Dublin, Ireland under the tradename Atlac 382. The fumed silicaused is commercially available under the trade name Aerosil R972 fromDegussa via Philips Duphar (Ireland) Limited, Dublin, Ireland.

[0091] In the examples the mixed adhesive composition is prepared usinga MIXPAC System 50 1:10 volume ratio dual syringe applicator and a 17stage static mixer nozzle, both commercially available from METIX (UK)Limited of Kettering NN16 8PX, England.

[0092] All of the tests on polypropylene substrates were carried out onfilled polypropylene (i.e. polypropylene PP-HWST supplied by Simona U.K.(Limited) of Stafford, England unless otherwise indicated. The term“natural polypropylene” is used for polypropylene which has no fillers.

[0093] In carrying out the Examples, no tendency to catch fire wasobserved during the handling of the metal alkyl borohydride initiators.

Example 1.

[0094] Initiator Component (Part A)

[0095] A lithium triethyl borohydride solution (2 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate (3 g).

[0096] Adhesive Component (Part B)

[0097] A slurry comprising a core-shell toughener (50 g), availablecommercially under the tradename Blendex 336 from GE SpecialityChemicals, tetrahydrofurfuryl methacrylate (125 g), 2-ethylhexylmethacrylate (42 g) and 2-acryloyloxyethyl maleate (34 g) was stirredwith a high shear mixer for several hours until a homogeneous dispersionwas obtained.

[0098] Adhesive

[0099] The initiator component A (5 g) and adhesive component B (45 g)were packaged respectively into the two syringes of a MIXPAC System 501:10 volume ratio 50 ml dual syringe applicator having a 1:10 volumeratio between the syringes holding the components A and B respectively.The two components were mixed by the simultaneous extrusion through a 17stage static mixer nozzle. The mixed adhesive composition was tested ona range of substrates as follows:

[0100] Samples of the adhesive were spread onto an untreated test-piece(4×1 inches) (101.6×25.4 mm) to form a film of approximately 0.1 mm indepth. Following which a second test-piece was brought against theadhesive to form an overlap adhesive joint with 0.5 inch (12.7 mm)overlap. A clamp was applied to the overlap area and the adhesive jointallowed to cure overnight, approximately 24 hours, at room temperature.The bond strengths for bonds assembled using the above procedure weretested on a Instron tester according to ASTM-D1002 and are presented inthe table below. Shear Strengths Substrates MPaPolypropylene/Polypropylene 4.2 +/− 0.5*

Example 2

[0101] Initiator Component (Part A)

[0102] A lithium triethyl borohydride solution (2 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 30% by weight ofPropoxylated Bisphenol-A Fumarate polyester(3 g).

[0103] Adhesives were prepared and evaluated using the aforementionedinitiator component and the adhesive component B from Example 1, to givethe following results. 3 Kg Fixture Shear Strengths times Substrates MPaMinutes Mild Steel/Polyethylene 3.75* 13 Mild Steel/Polypropylene 4.65*12 Polycarbonate/Polycarbonate 5.0 n/a Polypropylene/Polypropylene 4.1**11 Polyethylene/Polyethylene 3.3 12

[0104] The 3 Kg fixture times are the minimum cure times after bondassembly at which a bonded assembly will support a 3 Kg weight for aminimum of 10 seconds.

Example 3

[0105] Initiator Component (Part A)

[0106] The initiator component was prepared as outlined in Example 2.

[0107] Adhesive Component (Part B)

[0108] A slurry comprising a core-shell toughener Blendex 360 (100 g),tetrahydrofurfuryl methacrylate (249.5 g), 2-ethylhexyl methacrylate(83.2 g) and 2acryloyloxyethyl maleate (67.3 g) was stirred with a highshear mixer for several hours until a homogeneous dispersion wasobtained.

[0109] Adhesive

[0110] A mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in earlier examples. The bonds strengths for twosets of tests are presented in the table below. 3 Kg Fixture ShearStrengths times Substrates MPa Minutes (i) 0.5 inch (12.7 mm) overlapPolyethylene/Polyethylene 3.7* 12 Polypropylene/Polypropylene 4.2* 12.5Mild Steel/Polyethylene 3.2 10 Mild Steel/Polypropylene 3.2 10.5Polycarbonate/Polycarbonate 4.6 10.5 Mild Steel/Mild Steel 6.9 12

[0111] Shear Strengths Substrates MPa (ii) 0.25 inch (6.3 mm) overlapPolyethylene/Polyethylene 4.5 Polypropylene(filled)/ 2.6Polypropylene(filled) Polyethylene/Polycarbonate 3.0Glass/Polypropylene(filled) 5.3 Polycarbonate/Polypropylene(filled) 5.5ABS/Polypropylene(filled) 6.0 Zinc Bichromate/Polypropylene(filled) 5.0Glass/Polyethylene 5.2 ABS/ABS 8.0 Mild Steel/ABS 8.0 ZincBichromate/ABS 9.0 Nylon/Zinc Bichromate 4.4

Example 4

[0112] Initiator Component (Part A)

[0113] A lithium triethyl borohydride solution (2 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate (3 g) thickened with 30% by weightof Propoxylated Bisphenol-A Fumarate polyester.

[0114] Adhesive Component (Part B)

[0115] A slurry having the ingredients of Adhesive Component B ofExample 3 together with a micronised polyethylene powder (25 g)commercially available under the tradename Microthene FN500 distributedby National Chemical Company, Ireland, was stirred with a high shearmixer for several hours until a homogeneous dispersion was obtained.

[0116] Adhesive

[0117] A mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in earlier examples The bonds strengths arepresented in the table below. 3 Kg Fixture Shear Strengths timesSubstrates MPa Minutes Polyethylene/Polyethylene 4.2 13Polypropylene/Polypropylene 4.6* 12.0 Mild Steel/Polyethylene 4.5 11Mild Steel/Polypropylene 4.9* 11.5 Polycarbonate/Polycarbonate 3.8 9.5Mild Steel/Mild Steel 9.2 13

[0118] These results show significant improvements over Example 3, inwhich the polyethylene powder filler was not used.

Example 5

[0119] Initiator Component (Part A)

[0120] The initiator component was prepared as outlined in Example 4.

[0121] Adhesive Component (Part B)

[0122] The adhesive component was prepared as outlined in Example 3.

[0123] Adhesive

[0124] A mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as follows:

[0125] samples of the adhesive were spread onto an untreated test-pieces(4×1 inches—101.6×25.4 mm) to form a film of approximately 0.1 mm indepth. Following which a second test-piece was brought against theadhesive to form an overlap adhesive joint with 0.25 inch (6.4 mm)overlap. A clamp was applied to the overlap area and the adhesive jointallowed to cure overnight, approximately 24 hours, at room temperature.The bonds strengths for bonds assembled using the above procedure weretested on a Instron tester according to ASTM-D1002 and are presented inthe table below. Shear Strengths Substrates MPaPolyethylene/Polyethylene 8.0 Polypropylene/Polypropylene 7.9 MildSteel/Polyethylene 8.8 Mild Steel/Polypropylene 6.9 Mild Steel/EPDM 2.7Mild Steel/Glass (pins/Collar) 12.1*

Example 6

[0126] Initiator Component (Part A)

[0127] A lithium triethyl borohydride solution (2 g) (1 molar intetrahydrofuran) was mixed with the polyfunctional epoxy (3 g)commercially available from Ciba, Basle, Switzerland as DY-0396.

[0128] Adhesive Component (Part B)

[0129] The adhesive component was prepared as outlined in Example 3.

[0130] Adhesive

[0131] A mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in example 1 giving the following results ShearStrengths Substrates MPa Polyethylene/Polyethylene 1.5Polypropylene/Polypropylene 3.0 Mild Steel/Polyethylene 2.6 MildSteel/Polyproplene 4.1

Example 7

[0132] Initiator Component (Part A)

[0133] A lithium triethyl borohydride solution (10 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 30% by weight ofPropoxylated Bisphenol-A Fumarate polyester (15 g). The tetrahydrofuranwas removed by evaporation.

[0134] Adhesive Component (Part B)

[0135] The adhesive component was prepared as outlined in Example 3.

[0136] Adhesive

[0137] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in example 5, i.e. using the 0.25 inch (6.4 mm)overlap area The bonds strengths are presented in the table below. ShearStrengths Substrates MPa Polyethylene/Polyethylene 4.5Polypropylene/Polypropylene 4.7 Mild Steel/Polyethylene 6.7 MildSteel/Polypropylene 7.8 Polycarbonate/Polycarbonate 5.3

[0138] The results demonstrate that the presence of solvent in theinitiator component is not necessary for polyolefin adhesionperformance. Thus it is possible to prepare solvent-less polyolefinbonding adhesives, which have potentially significant environmental andlabelling advantages.

Example 8

[0139] Initiator Component (Part A)

[0140] A lithium tri-sec-butylborohydride solution (2 g) (1 molar intetrahydrofuran) commercially available under the tradenameL-Selectride® from Sigma-Aldrich Ireland Ltd. was mixed withtrimethylolpropane tris(3-(2-methylaziridino))propionate thickened with30% by weight of Propoxylated Bisphenol-A Fumarate polyester (3 g).

[0141] Adhesive Component (Part B)

[0142] The adhesive component was prepared as outlined in Example 3.

[0143] Adhesive

[0144] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in example 1 The bonds strengths for two sets oftests are presented in the table below. Shear Strengths Substrates MPa(i) 0.5 inch (12.7 mm) overlap Polyethylene/Polyethylene 2.1Polypropylene/Polypropylene 3.1 Mild Steel/Polyethylene 2.7 MildSteel/Polypropylene 2.3 Polycarbonate/Polycarbonate 3.35 Mild Steel/MildSteel 8.0 (ii) 0.25 inch (6.3 mm) overlap Polypropylene(natural)/ 3.9Polypropylene(natural) Glass/ABS 5.7* Polycarbonate/ 2.9Polypropylene(natural) ABS/Polypropylene(natural) 5.6 Polycarbonate/ABS4.3 Zinc Bichromate/ 6.2 Polypropylene(natural)Polyethylene/Polyethylene 2.6 Zinc Bichromate/Polyethylene 3.5 MildSteel/Polypropylene(natural) 8.9 Mild Steel/Polyethylene 5.0Glass/Polyethylene 2.9* ABS/Polyethylene 2.6Glass/Polypropylene(natural) 3.7* ABS/ABS 5.8 Mild Steel/ABS 9.3 ZincBichromate/ABS 10.4

Example 9

[0145] Initiator Component (Part A)

[0146] A lithium tri-sec-butylborohydride solution (2 g) (1 molar intetrahydrofuran) commercially available under the tradenameN-Selectride® from Sigma-Aldrich Ireland Ltd. was mixed withtrimethylolpropane tris(3-(2-methylaziridino))propionate thickened with30% by weight of Propoxylated Bisphenol-A Fumarate polyester (3 g).

[0147] Adhesive Component (Part B)

[0148] The adhesive component was prepared as outlined in Example 3.

[0149] Adhesive

[0150] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in example 1. The bonds strengths for two sets oftests are presented in the table below. Shear Strengths Substrates MPa(i) 0.5 inch (12.7 mm) overlap (Polyethylene/Polyethylene 2.8Polypropylene/Polypropylene 4.0 Mild Steel/Polyethylene 3.9 MildSteel/Polypropylene 2.6 Polycarbonate/Polycarbonate 3.1 Mild Steel/MildSteel 6.5 (ii) 0.25 inch (6.3 mm) overlap Polypropylene(natural)/ 4.6*Polypropylene(natural) Glass/ABS 4.8**Polycarbonate/Polypropylene(natural) 2.3* ABS/Polypropylene(natural)3.2** Polycarbonate/ABS 6.7** Zinc Bichromate/ 6.0*Polypropylene(natural) Polyethylene/Polyethylene 3.1** ZincBichromate/Polyethylene 2.0** Mild Steel/Polypropylene(natural) 3.7*Mild Steel/Polyethylene 3.8* Glass/Polyethylene 4.7** ABS/Polyethylene4.2** Glass/Polypropylene(natural) 3.8** ABS/ABS 8.0** Mild Steel/ABS8.2** Zinc Bichromate/ABS 9.4**

Example 10

[0151] Initiator Component (Part A)

[0152] A Sodium triethyl borohydride solution (2 g) (1 molar intetrahydrofuran) commercially available from Sigma-Aldrich Ireland Ltd.was mixed with trimethylolpropane tris(3-(2-methylaziridino))propionatethickened with 30% by weight of Propoxylated Bisphenol-A Fumaratepolyester (3 g).

[0153] Adhesive Component (Part B)

[0154] The adhesive component was prepared as outlined in Example 3.

[0155] Adhesive

[0156] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in earlier examples. The bonds strengths for twosets of tests are presented in the table below. Shear StrengthsSubstrates MPa (i) 0.5 inch (12.7 mm) overlap (Polyethylene/Polyethylene1.4 Polypropylene/Polypropylene 0.7 Mild Steel/Polyethylene 1.15 MildSteel/Polypropylene 2.0 (ii) 0.25 inch (6.3 mm) overlapPolyethylene/Polyethylene 1.9 Polypropylene(filled)/ 2.9Polypropylene(filled) Polypropylene(natural)/ 4.3 Polypropylene(natural)Midl Steel/Polypropylene(natural) 3.6 Mild Steel/Polypropylene(filled)3.5 Glass/Polypropylene(filled) 2.5 Polycarbonate/Polypropylene(filled)2.8 ABS/Polypropylene(filled) 2.8 Zinc Bichromate/Polypropylene(filed)6.5 Glass/Polyethylene 3.3 ABS/ABS 7.3 Mild Steel/ABS 6.0 ZincBichromate/ABS 7.8

Example 11

[0157] Initiator Component (Part A)

[0158] A Potassium triethyl borohydride solution (2 g) (I molar intetrahydrofuran) commercially available from Sigma-Aldrich Ireland Ltd.was mixed with trimethylolpropane tris(3-(2-methylaziridino))propionatethickened with 30% by weight of Propoxylated Bisphenol-A Fumaratepolyester (3 g).

[0159] Adhesive Component (Part B)

[0160] The adhesive component was prepared as outlined in Example 3.

[0161] Adhesive

[0162] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in earlier examples The bonds strengths for twosets of tests are presented in the table below. Shear StrengthsSubstrates MPa (i) 0.5 inch (12.7 mm) overlap Polyethylene/Polyethylene2.5 Polypropylene/Polypropylene 1.9 Mild Steel/Polyethylene 3.0 MildSteel/Polypropylene 2.9 Mild Steel/Mild Steel 9.5 (ii) 0.25 inch (6.3mm) overlap Polypropylene(natural)/ 3.2 Polypropylene(natural) Glass/ABS4.9 Polycarbonate/Polypropylene(natural) 2.2 ABS/Polypropylene(natural)2.4 Zinc Bichromate/ 3.4 Polypropylene(natural) ZincBichromate/Polyethylene(natural) 5.2 Mild Steel/Polypropylene(natural)3.4 Mild Steel/Polyethylene 3.8 Glass/Polypropylene(natural) 1.7 ABS/ABS5.2 Mild Steel/ABS 5.4 Zinc Bichromate/ABS 4.3

Example 12

[0163] Initiator Component (Part A)

[0164] A Lithium 9-BBN (borobicyclo nonane) hydride solution (2 g) (1molar in tetrahydrofuran) commercially available from Sigma-AldrichIreland Ltd. was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 30% by weight ofPropoxylated Bisphenol-A Fumarate polyester (3 g).

[0165] Adhesive Component (Part B)

[0166] The adhesive component was prepared as outlined in Example 3.

[0167] Adhesive

[0168] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in Example 5. The bond strengths are presented inthe table below. Shear Strengths Substrates MpaPolypropylene/Polypropylene 0 Polyethylene/Polyethylene 0 MildSteel/Polypropylene 0 Mild steel/Polyethylene 0 Mild Steel/Mild Steel9.4 Polycarbonate/Polycarbonate 3.1

[0169] These results show that when the alkyl groups of the initiatorform a cyclic structure, the adhesive polymerises well and bonds steelbut has poor adhesion to polyolefins.

Example 13

[0170] Initiator Component (Part A)

[0171] A lithium triethyl borohydride solution (0.5 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 30% by weight ofPropoxylated Bisphenol-A Fumarate polyester (3 g).

[0172] Adhesive Component (Part B)

[0173] The adhesive component was prepared as outlined in Example 3.

[0174] Adhesive

[0175] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in Example 1. The bonds strengths are presentedin the table below. Shear Strengths Substrates MPaPolyethylene/Polyethylene 3.8 Polypropylene/Polypropylene 3.0

Example 14

[0176] Initiator Component (Part A)

[0177] A lithium triethyl borohydride solution (1.0 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 30% by weight ofPropoxylated Bisphenol-A Fumarate polyester (3 g).

[0178] Adhesive Component (Part B)

[0179] The adhesive component was prepared as outlined in Example 3.

[0180] Adhesive

[0181] The mixed adhesive composition was prepared as outlined inExample 1. The mixed adhesive composition was tested on a range ofsubstrates as outlined in Example 1. The bond strengths are presented inthe table below. Shear Strengths Substrates MPaPolyethylene/Polyethylene 4.9 Polypropylene/Polypropylene 4.8 MildSteel/Polyethylene 4.7 Mild Steel/Polypropylene 4.8

Example 15

[0182] Initiator Component (part A)

[0183] A lithium triethyl borohydride solution (3.0 g) (1 molar intetrahydrofuran) was mixed with trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 30% by weight ofPropoxylated Bisphenol-A Fumarate polyester (3 g).

[0184] Adhesive Component (Part B)

[0185] The adhesive component was prepared as described in Example 3.

[0186] Adhesive

[0187] The initiator component A (5 g) and adhesive component B (45 g)were mixed as described in Example 1. The mixed adhesive composition wastested on a range of substrates as outlined in example 1. The bondsstrengths are presented in the table below. Shear Strengths SubstratesMPa Polyethylene/Polyethylene 1.9 Polypropylene/Polypropylene 4.2Polyethylene/ 5.2 Polypropylene(unfilled) Mild Steel/Polyethylene 4.7Mild Steel/Polypropylene 4.9 Mild Steel/Polypropylene(unfilled) 4.3 PTFE1.0

Example 16

[0188] Example 8 was followed except that the initiator compound usedwas lithium trisiamylborohydride which is commercially available underthe tradename LS Selectride® from Sigma-Aldrich Ireland Ltd. The bondstrengths from tests with 0.25 inch (6.3 mm) overlap were as follows:Shear Strengths Substrates MPa Polypropylene/Polypropylene 0.5Polyethylene/Polyethylene 0.4 Mild Steel/Polypropylene 0.5 MildSteel/Polyethylene 0.8 Mild Steel/Mild Steel 8.0

[0189] These results illustrate the effectiveness of the initiator in acomposition which polymerises well and bonds steel but shows that whenthe alkyl groups on the boron atom are longer or more branched thanstraight-chain C₁-C₄ alkyl there is a significant reduction ofpolyolefin bonding capability.

Example 17

[0190] Samples of potassium tri-sec-butyl borohydride in tetrahydrofuran(commercially available) and in diglyme and dibutyl ether were obtainedfrom Callery Chemicals, and were compared with potassium tri-sec-butylborohydride commercially available under the tradename K-Selectride fromSigma-Aldrich Ireland Ltd. Example 11 was followed except that theseinitiator compounds were used. The bond strengths for tests with 0.25inch (6.23 mm) overlaps were as follows: Bond Strengths MPa Tri alkylborohydride salt PE/GBMS PE/PE PP/PP Calselect K (K tri sec-butyl 5.72.2 1.8 BH in THF) Calselect K (K tri sec-butyl 5.3 2.7 2.1 BH inDiglyme) Calselect K (L tri sec-butyl 5.8 5.2 2.9 BH in Bu₂O)K-Selectride (K tri sec- 4.5 2.7 3.1 butyl BH in THF) (Aldrich)

Example 18

[0191] Initiator Component (Part A)

[0192] Lithium triethylborohydride 1 molar in THF (Super-Hydride fromAldrich) (2 g) was added to trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 5% by weight offumed silica Aerosil R972 manufactured by Degussa (3 g).

[0193] Adhesive Component (Part B)

[0194] A slurry comprising a core-shell toughener (100 g), availablecommercially under the tradename Blendex B336 from GE SpecialtyChemicals, tetrahydrofurfuryl methacrylate 249.5 g), 2-ethylhexylmethacrylate (83.2 g) and 2-acroyloyloxyethyl maleate (67.3 g) wasstirred with a high shear mixer for several hours until a homogeneousdispersion was obtained.

[0195] Adhesive

[0196] The adhesive component A (5 g) and component B (45 g) werepackaged respectively into a MIXPAC System 50 1:10 volume ratio dualsyringe applicator commercially available from METIX (UK) Limited.Several samples of the packaged adhesives were placed in temperaturecontrolled environments at 35° C. and at monthly intervals one of thepackages was removed and tested. The two components were mixed by thesimultaneous extrusion through a 17 stage static mixer nozzle alsoavailable from METIX (UK) Ltd. The mixed adhesive composition was testedon a range of substrates as outlined in Example 5. The bond strengthsare presented in the table below. Shear Strengths MPa after ageing at35° C. Substrates 1 month 2 months 3 months 4 months 5 monthsPolyethylene/ 6.6 2.4 3.2 3.2 5.2 Polyethylene Polypropylene/ 3.5 5.05.0 3.1 5.2 Polypropylene

Example 19

[0197] Initiator Component (Part A)

[0198] Lithium triethylborohydride 1 molar in THF (Super-Hydride fromAldrich) (2 g) was added to trimethylolpropanetris(3-(2-methylaziridino))propionate thickened with 5% by weight offumed silica Aerosil R972 manufactured by Degussa (3 g).

[0199] Adhesive component (Part B)

[0200] Adhesive compositions were prepared with varying levels of acidingredients as outlined below: 0% 5% 10% 20% Acid Acid Acid Acid BlendexB360 100 g 100 g 100 g 100 g tetrahydrofurfuryl 249 g 249 g 249 g 249 gmethacrylate 2-ethylhexyl 150.5 g 124.5 g 100.5 g 50.5 g methacrylate2-acryloyloxyethyl 0 g 25 g 50 g 100 g maleate

[0201] Adhesive

[0202] Mixed adhesive compositions were prepared as in Example 1. Themixed adhesive compositions were tested on a range of substrates asoutlined in Example 5. The bond strengths are presented in the tablebelow. Bond Strengths/MPa for Acid levels Substrates 0% 5% 10% 20%Polyethylene/Polyethylene 1.7 3.4 4.4 2.1 Mild Steel/Polypropylene 2.04.5 4.2 3.0 Mild Steel/Mild Steel 2.5 1.5 7.5 8.0

[0203] A range of carboxylic acids were screened and found to be active,for example replacing 2 acryloyloxyethyl maleate with either acrylicacid or methacrylic acid had little effect on the adhesive performance.Generally the simple polymerisable carboxylic acids are preferred.

1. Use of metal alkyl borohydrides of the formula I or II:

wherein R¹ is C₁-C₁₀alkyl, R² and R³, which may be the same ordifferent, are H, D, C₁-C₁₀ alkyl or C₃-C₁₀ cycloalkyl, phenyl, orphenyl-substituted C₁C₁₀ alkyl or C₃-C₁₀ cycloalkyl, provided that anytwo of R¹-R³ may optionally be part of a carbocyclic ring, and M⁺ is ametal ion, as initiators of polymerisation.
 2. Use according to claim 1of metal alkyl borohydrides of the formula I or II as initiators ofpolymerisation in adhesive compositions.
 3. Use according to claim 1 or2 wherein the metal alkyl borohydride is of the formula I.
 4. Useaccording to any of the preceding claims wherein the metal alkylborohydride is of the formula I or II in which M⁺ is an alkali metalion.
 5. Use according to any of the preceding claims wherein the metalalkyl borohydride is an alkali metal trialkyl borohydride.
 6. Useaccording to any of the preceding claims wherein the initiator compoundis of formula I in which each of R¹-R³ is the same C₁-C₁₀ alkyl group.7. Use according to any of the preceeding claims wherein the initiatorcompound is of formula I in which each of R¹-R³ is the same as C₂-C₄alkyl group.
 8. Use according to claim 1 wherein the metal salt isselected from the group consisting of Lithium triethylborohydride,Sodium triethylborohydride, Potassium triethylborohydride, Lithiumtri-sec-butylborohydride, Sodium tri-sec-butylborohydride, Potassiumtri-sec-butylborohydride, Lithium 9-borabicyclo [3.3.1]-nonane (9-BBN)hydride, Lithium thexylborohydride, Lithium trisiamylborohydride,Potassium trisiamylborohydride, and Lithium triethylborodeuteride. 9.Use according to claim 1 wherein the metal salt is selected from theGroup consiting of: Lithium triethylborohydride, Sodiumtriethylborohydride, Potassium triethylborohydride, Lithiumtri-sec-butylborohydride, Sodium tri-sec-butylborohydride, Potassiumtri-sec-butylborohydride and Lithium triethylborodeuteride.
 10. Apolymerisable composition comprising: a) at least one free-radicallypolymerisable monomer component, and b) an effective amount of aninitiator system for initiating polymerisation of the free-radicallypolymerisable monomer, said initiator system comprising a metal alkylborohydride of the formula I or II:

 wherein R¹ is C₁-C₁₀alkyl, R² and R³, which may be the same ordifferent, are H, D, C₁-C₁₀ alkyl or C₃-C₁₀ cycloalkyl, phenyl, orphenyl-substituted C₁-C₁₀ alkyl or C₃-C₁₀ cycloalkyl, provided that anyof R¹-R³ may optionally be part of a carbocyclic ring, and M⁺ is a metalion.
 11. A composition according to claim 10 which is a two-partcomposition in which the free-radically polymerisable monomer componentis provided in one part and the metal alkyl borohydride is provided inthe other part. 12 A two-part polymerisable adhesive compositionaccording to claim 10 comprising: part A) an effective amount of apolymerisation initiator comprising the metal alkyl borohydride, and acarrier inert to the metal alkyl borohydride; and part B) at least one(meth)acrylate monomer, optionally with a toughener, acidic monomer,filler or thickener.
 13. A composition according to any of claims 10-12wherein the metal alkyl borohydride is of the formula I.
 14. Acomposition according to any of claims 10-12 wherein the metal alkylborohydride is of formula I or II in which M⁺ is an alkali metal ion.15. A composition according to any of claims 10-14, wherein each ofR¹-R³ is a C₂C₆ alkyl group.
 16. A composition according to any ofclaims 10-15 wherein each of R¹-R³ is the same C₂-C₄ alkyl group.
 17. Acomposition according to any of claims 10-15, wherein the metal salt isselected from the group consisting of: Lithium triethylborohydride,Sodium triethylborohydride, Potassium triethylborohydride, Lithiumtri-sec-butylborohydride, Sodium tri-sec-butylborohydride, Potassiumtri-sec-butylborohydride, Lithium 9-borabicyclo [3.3.1]-nonane (9-BBN)hydride, Lithium thexylborohydride, Lithium trisiamylborohydride,Potassium trisiamylborohydride, and Lithium triethylborodeuteride.
 18. Acomposition according to any of claims 10-15 wherein the metal salt isselected from the group consisting of: Lithium triethylborohydride,Sodium triethylborohydride, Potassium triethylborohydride, Lithiumtri-sec-butylborohydride, Sodium tri-sec-butylborohydride, Potassiumtri-sec-butylborohydride and Lithium triethylborodeuteride.
 19. Acomposition according to any of claims 10-18 for bonding a low surfaceenergy substrate to a similar or different substrate.
 20. A compositionaccording to any of claims 10-19 for bonding a polyolefin substrate to asimilar or different substrate.
 21. A composition according to any ofclaims 10-20, further comprising a polyolefin filler.
 22. A compositionaccording to any of claims 10-21 further comprising anaziridine-functional material as a carrier for the metal alkylborohydride.
 23. A composition according to any of claims 10-22 furthercomprising a core-shell polymer toughener.
 24. A composition accordingto any of claims 10-23, wherein the free-radically polymerisable monomercomponent comprises a (meth)acrylic monomer.
 25. A composition accordingto claim 24, wherein the free-radically polymerisable monomer componentcomprises at least two (meth)acrylic monomers.
 26. A compositionaccording to any of claims 10-25, wherein the quantity of metal alkylborohydride is such as to provide 0.01% to 5% by weight, particularly0.01% to 2% by weight, such as 0.01% to 0.6% by weight, of boron in thetotal composition.
 27. A composition according to any of claims 10-25which includes an acid in the polymerisable monomer component.
 28. Acomposition according to claim 27 wherein the acid has a pKa from 0.5 toabout
 13. 29. A composition according to claim 27 or 28 wherein the acidis an acidic monomer.
 30. A composition according to any of claims 10-29wherein at least a portion of the metal alkyl borohydride is a 1:2adduct of the formula Ia or IIa:

wherein R¹, R², R³, and M⁺ are as defined above.
 31. A method forbonding a substrate, particularly a low surface energy substrate such asa polyolefin substrate, to a similar or different substrate, wherein themethod comprises applying a composition according to any of claims 10-30to at least one of the substrates, bringing the substrates together withthe composition between them and allowing the composition to cure.
 32. Amethod according to claim 31, wherein the method comprises mixing partsA and B of a two part adhesive composition immediately prior to use inorder to initiate polymerisation, applying the mixed adhesivecomposition to at least one of the substrates, bringing the substratestogether and allowing the composition to cure by completion of thepolymerisation initiated on mixing of the two parts A and B.
 33. Amethod of initiating polymerisation in a free-radically polymerisablemonomer which comprises mixing the monomer with a metal alkylborohydride of the formula I or II as defined in claim 1, optionally inthe presence of an added acid.
 34. A method according to claim 33wherein the acid is an acidic monomer which is free-radicallypolymerisable.
 35. A cured adhesive composition comprising thepolymerisation product of a composition according to any of claims10-30.
 36. A bonded article comprising two substrates bonded by thepolymerisation product of a composition according to any of claims10-30.
 37. A bond formed between two substrates by the polymerisationproduct of a composition according to any of claims 10-30.
 38. Apolymerisable composition, which includes a) at least one free-radicallypolymerisable monomer component, b) an effective amount of an initiatorsystem for initiating polymerisation of the free-radically polymerisablemonomer, said initiator system comprising a metal alkyl borohydride ofthe formula I or II as defined in claim 1, and c) an effective amount ofa compound that is reactive with the metal alkyl borohydride to free anorganoborane from the metal alkyl borohydride and thus to initiatepolymerisation of the at least one free-radically polymerisable monomer.39. A system capable of initiating the polymerisation of a (meth)acrylicmonomer, the system comprising: a) a metal alkyl borohydride of theformula I or II as defined in claim 1; and b) an effective amount of acompound that is reactive with the metal alkyl borohydride to free anorganoborane from the metal alkyl borohydride.
 40. A polymerisableacrylic composition comprising: a) a (meth)acrylic component comprisingat least one (meth)acrylic monomer; b) an effective amount of a metalalkyl borohydride of the formula I or II as defined in claim 1; and c)an effective amount of a compound that is reactive with the metal alkylborohydride to free an organoborane from the metal alkyl borohydride andthus to initiate polymerisation of the at least one (meth)acrylicmonomer.
 41. A composite article comprising a first substrate and asecond substrate bonded to the first substrate by an acrylic adhesive,wherein the acrylic adhesive comprises the polymerisation product of apolymerisable acrylic composition that comprised: a) a (meth)acryliccomponent comprising at least one (meth)acrylic monomer; b) an effectiveamount of a metal alkyl borohydride of the formula I or II as defined inclaim 1, and c) an effective amount of a compound that is reactive withthe metal alkyl borohydride to free an organoborane from the metal alkylborohydride and thus to initiate polymerisation of the at least one(meth)acrylic monomer.
 42. A method of bonding a low surface energypolymer to a substrate, the method comprising the steps of: i) providinga low surface energy polymer; ii) providing a substrate; iii) providingan adhesive composition comprising: a) at least one (meth)acrylicmonomer; b) an effective amount of a metal alkyl borohydride of theformula I or II as defined in claim 1; and c) an effective amount of acompound that is reactive with the metal alkyl borohydride to free anorganoborane from the metal alkyl borohydride and thus to initiatepolymerisation of the at least one (meth)acrylic monomer. iv) applyingthe adhesive composition to either the low surface energy polymer or thesubstrate; v) joining the low surface energy polymer and the substratewith the adhesive composition therebetween; and vi) permitting theadhesive composition to cure to adhesively bond the low surface energypolymer and the substrate.
 43. A method according to any of claims 31-34or 42 which is carried out in the presence of air or oxygen.